The present invention relates to an automatic gain control (AGC) and, more particularly, to an AGC for a demodulator which receives digitally modulated burst signals from a plurality of stations.
The time division multiple access (TDMA), PURE ALOHA and SLOTTED ALOHA communication systems, for example, belong to a family of multiple access satellite communication systems which set up communications between a single central earth station and a plurality of remote earth station via a satellite over a common channel. In such a multiple access satellite communication system, the central earth station receives digitally modulated burst signals lying in the radio frequency band time-serially from the remote earth stations over a common channel. The burst signals lying in the radio frequency band are converted into intermediate frequency modulated waves. To allow a demodulator to demodulate the modulated waves, it is necessary that the reception power of the burst signals from the remote earth stations be constant. Two different methods are available for meeting the above requirement, i.e., a method that causes each remote earth station to control the transmission power of the burst signal to be transmitted and a method that causes the central earth station to automatically control the gain of each received burst signal.
The Very Small Aperture Terminal (VSAT) system or similar system which needs miniature and inexpensive remote earth stations is implemented only with the above-mentioned central earth station-oriented method.
An AGC for maintaining the reception power at the central earth station constant may be implemented as a closed loop of variable attenuator, demodulator, comparator, and low-pass filter, as described in pages 381-385 of a book entitled "Digital Transmission SYSTEMS" by P Bylanski and DGW Ingram and published by peter Peregrinus Ltd., reprinted 1987. The demodulator receives digitally modulated signals from the remote earth stations via the variable attenuator, and then demodulates them to produce demodulated signals. The comparator calculates the power levels from the demodulated signals, compares the results of calculation with a reference power, and then outputs signals representative of the differences. The low-pass filter removes noise from the outputs of the comparator. The variable attenuator adjusts the amounts of attenuation of the modulated waves in response to control signals which are the outputs of the low-pass filter.
In the closed loop type AGC system, when the reception power is changed, the response speed up to the time when desired output power is achieved is inversely proportional to the equivalent noise bandwidth of the closed loop. The problem with the satellite communication system is that the C/N ratio of the demodulator is so low that the equivalent noise bandwidth has to be reduced, resulting in low response speed.
Usually, butst signals from remote earth stations each is headed by a preamble word which is a training bit sequence for recovering a carrier and a bit timing. To faithfully recover a data signal following the preamble word, the reception power should preferably be controlled in the preamble word portion.
However, the reception power sometimes greatly differs from one burst signal to another received by the central earth station due to the scattering among the transmission outputs of the individual remote earth stations, different weather conditions, etc. In this condition, when the remote earth stations send burst signals in a single frame on a time division basis, the reception power or level at the central earth station greatly differs from one burst signal to another. Hence, it is extremely difficult for the conventional AGC to maintain the reception power constant in the preamble word portion due to the slow response particular thereto. Should the power be not made constant in the preamble word portion, the central earth station would receive burst signals incorrectly or, if received them correctly, degrade the bit error rate (BER) in the data signal portion.